Most people here don't understand what monopoly actually means.
You don't need to be the only player, it's enough to have sufficient market share. Otherwise google wouldn't have been subject to regulation in the past. Everyone can choose a different search engine if they want to. However, we are not only talking about users, but also the other side. In case of google: the websites. If your business isn't found using google, what happens? Loss of revenue.
This is not about iPhone users, but developers!
If that's not clear enough: as a dev you are not free to choose the plattform. If you don't have an App on the Play Store and the App-Store your app revenues will be ZERO.
Although, for Android you can upload the APK to your website and provide the download link upon payment if google decides to kick you out.
How do you that for iPhones? XCode? Only with open source. Now tell me again how Apple does not qualify as monopolist.
The App-Store is the only way devs can reach customers. However, Apple is exercising a policy on the App Store where it won't publish Apps of similar behaviour or for whatever other arbitrary reason (see Steam). In it's current state the App-Store is not a capitalistic market place, but Apples app-selection. Also, when you re-install you phone, your purchased App my no longer be available as Apple removed it from the App-Store.
As more plattforms implement this store concept it requires a legal framework. Especially when it comes to change of Terms & Conditions. New T&C can't be forced upon customers that have made a purchase without the possiblity of a refund (i.e. 100% in the first year, 50% in the 2nd and 25% in the 3rd year). Currently, this "here, have our new T&Cs! If you don't like them you may quit and lose all your paid apps." is a bad joke.
Also a store provider must not delete any purchased app without due notice to make a local backup.
A store provider must publish all apps if they fullfill plattform requirements & pass QC and security checks.